This invention relates generally to metal object detection and more particularly to an inductive loop detector suitable for detecting the passage or presence of a vehicle over a defined area of roadway.
Inductive loop detectors have been widely used for several years in various applications to detect the presence or passage of a vehicle. For example, such detectors have been used in traffic actuated control systems for developing the input data required by a controller to control signal lights. In another typical application, a detector may be connected to a counter which functions to accumulate a count of vehicle flow past a certain point.
Various types of inductive loop detectors suitable for traffic applications are known in the prior art. Each detector generally comprises electronic circuitry which operates in conjunction with a loop (i.e., a wire coil) buried in the roadway in a plane substantially parallel to the roadway surface. The circuitry includes components which, together with the loop, form an oscillator whose frequency is dependent on the loop inductance. The loop inductance is in turn dependent on whether or not the loop is loaded by the presence of a vehicle. A vehicle over the loop decreases loop inductance and thus increases the frequency of oscillation. The circuitry monitors the oscillator frequency and generates a call (i.e., vehicle present signal) when a sufficient frequency change is detected. Different techniques have been employed in prior art detectors for monitoring the oscillator frequency but generally, analog circuitry using filters and phase detectors has been employed.
U.S. Pat. application Ser. No. 444,519 now abandoned, assigned to the same assignee as the present application discloses an improved inductive loop detector which employs digital circuitry to monitor the inductance of a loop. The digital circuitry defines sequential detect cycles and during each detect cycle, a digital counter (sometimes referred to as the loop counter) counts cycles of the loop oscillator signal. Concurrently, a second digital counter (sometimes referred to as the duration counter) measures the duration or period of a predetermined number of loop signal cycles by counting pulses provided by a stable clock pulse source. The measured duration is then compared with a reference duration (whose value is accumulated during prior detect cycles in accordance with predetermined criteria) and the difference is indicative of a change in loop signal frequency and thus also a change in loop inductance. A threshold means then determines whether the change is of sufficient magnitude to generate a call. In accordance with an important aspect of the detector disclosed in Ser. No. 444,519 now abandoned, the difference (sometimes called deviation) between the measured and reference durations is utilized as an error input to modify (i.e., servo) the reference duration toward the measured duration to thus allow the detector to self-tune or adapt to varying environmental conditions. The rate of adapting is dependent on the magnitude and direction of the deviation.